1. Field of the Invention
The present invention is directed to a system and method for improving the resolution and tissue contrast in MRI.
2. Description of Related Art
The best current source of raw image data for observation of a complex soft tissue and bone structure is magnetic resonance imaging (MRI). MRI involves the transmission of RF signals of predetermined frequency (e.g., approximately 15 MHZ in some machines, the frequency depending upon the magnitude of magnetic fields employed and the magnetogyric ratio of the atoms to be imaged). Typically, exciting pulses of RF energy of a specific frequency are transmitted via an RF coil structure into an object to be imaged. A short time later, radio-frequency NMR responses are received via the same or a similar RF coil structure. Imaging information is derived from such RF responses.
In MRI, a common imaging technique is the formation of images of selected planes, or slices, of the subject being imaged. Typically the subject is located in the static magnetic field with the physical region of the slice at the geometric center of the gradient field. Generally, each gradient will exhibit an increasing field strength on one side of the field center, and a decreasing field strength on the other side, both variations progressing in the direction of the particular gradient. The field strength at the field center will thus correspond to a nominal Larmor frequency for the MRI system, usually equal to that of the static magnetic field. The specific component of a gradient which causes the desired slice to be excited is called the slice selection gradient. Multiple slices are taken by adjusting the slice selection gradient.
However, MRI often introduces the following technical challenges. Many of the anatomical structures to be visualized require high resolution and present low contrast, since, for example, many of the musculoskeletal structures to be imaged are small and intricate. MRI involves the use of local field coils to generate an electromagnetic field; such local field coils form a non-uniform illumination field. MRI images can also be noisy.
In particular, MRI has the following limitations in resolution and tissue contrast. Although current MRI machines can achieve relatively high intra-plane resolution, the inter-slice resolution is not so good as the intra-plane resolution; also, the inter-slice resolution is limited by the ability of the system to stimulate a single spatial slice or section. Although tissue contrast can be adjusted by selecting the right pulse sequence, analysis of a single pulse sequence is not enough to differentiate among adjacent similar tissues. In other words, the resolution is typically poor in the out-of-plane dimension, and the contrast is typically low between soft tissue structures.